8020 doesn't recognize external usb disk

Hi friends, i've two external usb disk (by Packard Bell) and i've tried to connect them to the Spd8020. In both case the Nas doesn't recognize the usb disk.
I've also tried to connect a usb pen drive and another external usb disk (interface by Magnex) and the nas recognize them perfectly.
I've updated the firmware to the last version (8-0). What can i try to resolve the problem?
Thanks
I've also tried to connect a usb pen drive and another external usb disk (interface by Magnex) and the nas recognize them perfectly.
I've updated the firmware to the last version (8-0). What can i try to resolve the problem?
Thanks